Germanium-containing compounds are known to have therapeutic and prophylactic properties when introduced to the human body. For example, use of a specific compound of germanium, carboxyethylgermanium sesquioxide (Ge.sub.cD6 H.sub.12 O.sub.7), to supplement the human diet has been shown to have chemotherapeutic value. [Asai, K., Miracle Cure: Organic Germanium, Japan Publications, Inc., Tokyo (1980) ("hereinafter Asai")]. It is also known that carboxyethylgermanium sesquioxide can provide the human body with resistance to the common cold. [See, Nakao Ishida, et. al., U.S. Pat. No. 4,595,882]. Carboxyethylgermanium sesquioxide is also used as a dietary supplement for treatment of hypertension. [See, Asai, U.S. Pat. No. 3,793,455]. Stimulating the human immune system, and more particularly stimulating the production of interferon, are also believed to be important roles of carboxyethylgermanium sesquioxide. [See, Asai].
A drawback to using some forms of germanium as a food supplement or chemotherapeutic agent is the inherent toxicity attributed to inorganic germanium compounds, such as germanium dioxide (GeO.sub.2) and metallic germanium. In contrast to such forms of germanium, carboxyethylgermanium sesquioxide is non-toxic to the human body, having LD50 values in excess of 5 g/kg.
There are two major drawbacks of known techniques for the dietary use of carboxyethylgermanium sesquioxide. First, it is difficult to obtain the compound in pure form. Since carboxyethylgermanium sesquioxide is a completely synthetic compound, and since most syntheses start with a form of germanium that is toxic to the human body, the ultimate purity of the synthesized product, carboxyethylgermanium sesquioxide, can be in question--especially when produced on a large scale. Second, carboxyethylgermanium sesquioxide produced by known techniques requires high dosage levels. Effective dosage levels for dietary supplementation of carboxyethylgermanium sesquioxide generally range from 15 mg/day to over 1 g/day. [See, Asai, at 55-65]. These high doses of the compound are considered to be necessary since the amount of carboxyethylgermanium sesquioxide that is actually metabolized and used constructively by the human body is unknown. It is conventionally believed that a rather large dosage level is important for maximum benefit, suggesting that an appreciable amount of the ingested germanium is unused by the human body. [See, Asai, at 38-39].
Forms of germanium derived from yeast are believed to be of greater value as dietary supplements or chemotherapeutic agents since these forms of germanium are more readily utilized by the body. Since yeast-derived forms of germanium are possibly more capable of being metabolized than other forms of germanium, less yeast-derived germanium needs to be ingested in order to realize the germanium's desired health benefits.
Yeast preparations containing metal ions are not new. A method of producing yeast-derived sources of metabolizable chromium has been reported. [See, e.g., Skogerson, U.S. Pat. No. 4,348,483]. Beneficial aspects of chromium yeast, and their superior nutritional properties in comparison to inorganic forms of chromium have been established. [See, e.g., Shepard, Biol. Trace Elem. Res., 32:109-113 (1992), reporting the use of chromium yeast for potentiation of insulin action in adipocytes from rats]. Selenium metal, usually highly toxic when administered in inorganic form such as SeO.sub.3 Na.sub.2, has been shown to be more effective nutritionally when ingested as a yeast-derived substance. [See, e.g., Baerwald, Gordian, 94(11):169-173 (1994)].
A method for preparing germanium-derived yeast has been taught by Komatsu, JP 77-46138770420. However, this method involves the preparation of germanium yeast using a highly toxic form of germanium, GeO.sub.2, as the source of germanium for the feed and cultivation of the yeast. The major shortcoming of the method of Komatsu is that the yeast produced by the method has an appreciable content of highly toxic non-metabolizable germanium, and is therefore not useful as a human dietary supplement.
There remains a need in the art for an improved method for the preparation of germanium yeast where: (1) the yeast product is essentially non-toxic to the human body; and (2) the chemical form of germanium produced by the germanium yeast is highly metabolizable by the human body, and thus a useful and significantly improved agent for the nutritional supplementation of the human diet, and for chemotherapeutic uses.